Lunar economics etc

SpaceNut · 2019-03-19 17:00:37

In that regards all nations cooperate as they do now on earth. Until the on earth area changes in that regards no change will be possible to that same phylosophy for all outer space locations.
The plan it seems has or is being revised for SLS and the LOP-G Stations transistion to corporately being built and launched for the most part for the next decade. Administration proposes the end of EUS while Administrator considers full Exploration manifest rewrite

Terraformer · 2019-03-20 03:38:01

Legally speaking, what counts as part of a base on another world? If it's a complex of buildings, are the bits in between counted, which would prevent other organisations from setting up in the middle of you?

I suppose the closest analogue today would be a ship trying to place itself in the middle of a convoy without permission. What is the legality of that move? I know there's a (500m?) buffer zone around each vessel. Presumably there will have to be something like this for off-world bases, too.

If the buffer zone around a spacecraft is bigger than the asteroid you're trying to mine... well, no-one else will be able to mine it without breaking admiralty law.

tahanson43206 · 2019-03-20 09:42:17

Bravo, Terraformer!

If you happen to discover someone who would be willing (and qualified) to address your question, we are set up to accept a Guest Speaker.

Notify SpaceNut if you have someone you would like to bring onto the forum to discuss this issue/these issues.

As a suggestion, you will find several lawyers who specialize in space law in the guest list of The Space Show. Most provide contact information.

(th)

Terraformer wrote:

Legally speaking, what counts as part of a base on another world? If it's a complex of buildings, are the bits in between counted, which would prevent other organisations from setting up in the middle of you?
I suppose the closest analogue today would be a ship trying to place itself in the middle of a convoy without permission. What is the legality of that move? I know there's a (500m?) buffer zone around each vessel. Presumably there will have to be something like this for off-world bases, too.
If the buffer zone around a spacecraft is bigger than the asteroid you're trying to mine... well, no-one else will be able to mine it without breaking admiralty law.

SpaceNut · 2019-03-20 19:14:49

Could also dig underground causing collapse of structures as well wanting minerals. This is just being a bad neighbor and not something we need to bring with us.

SpaceNut · 2019-05-10 20:10:06

Something that we have talked about in what mass and energy would be required for the machine to make it possible to live underground.

Lunar tunnel engineers excited by boring Moon colonies

top engineers are racing to design a tunnel boring machine capable of digging underground colonies for the first lunar inhabitants.

Thats good news as that would also apply to mars which would maybe give Musk a push to work on that as wel as a BFR...

"Imagine something the size of my fist as a piece of rock coming at 10-12 kilometres (6-7 miles) per second, it can hit anything and would immediately destroy it

ouch but we can not be chicken little so lets plant to be a bit deeper so as to be able to survive such an impact.

four-metre diameter machine needing some 2,000 kilowatts of energy, experts are debating whether it is possible to use small nuclear power plants to fuel a lunar version,

That puts it in the range for being powered by the nasa kilowatt nuclear reactors and it can be all autonomous with tele-robotic interface.

kbd512 · 2019-05-10 21:43:34

SpaceNut,

We could deploy a 2MW solar array that would continuously produce full output for two weeks at a time. The thin film panels are already 1kW/kg+, so if we added sufficient support structure, maybe we could drag that down to 500W/kg. That's still just 4t for 2MW of power.

The Robbins high performance 3.5m TBM's weigh 180t and require 1.34MW worth of input power. Speaking of "thrust", here's one for our rocket boys. The 3.5m TBM generates 7,800kN of thrust, or about the same as an up-rated F1 engine. The "best rate of advance" for one of Robbins' 4.3m machines was 312m / 1,024ft in a single week. They only need a crew of 4 operators (one machine operator / rail car loader, one locomotive operator, one electrician, and one mechanic). I'm guessing we can knock that back to 2 operators with modern automation technology. That was in the late 1980's / early 1990's.

Say we only get half of that. That's still 150m per week and 300m per month. Why send a single machine? These things are usually deployed in pairs. How about 2 machines for double the price? That's enough well-protected living space to house a small army of colonists. Combined with CNT inflatables technology, we could easily provision enough pressurized living space to deliver a division's worth of young men and women to the moon every month. We'd need a small army to keep the machines running, anyway. We could use smaller access tunnels to deliver natural and artificial sunlight to plants using small orbital laser arrays to deliver concentrated sunlight from orbit during the two weeks of darkness to keep the crops alive and the people sane.

SpaceNut · 2019-05-11 08:22:13

Not that solar on the moon would be bad its the impacts all of which we have witnessed and the panels would not survive when made so large as to not be able to get them moved in time since we have not had all that much time to impact for such events.

The boring machine will need to be untethered from any power source as it will need to operate 24x7 to make it easy to not worry about the lunar day and night. That does not mean we can not use battery storage onboard but it also means long before its discharged that the machine must make its way back to the charging system. A mobile recharging station that runs behind the boring machine might work buts thats added complexity and mass to the moon.

louis · 2019-05-11 08:38:36

I feel that we are not a naturally subterranean species and that living on the surface is much suited to homo sapiens. I consider tourism to be the gateway sector for human occupation of the moon, and that in particular will demand a surface experience I think.

So, as on Mars, I favour surface habitats, probably delivered whole from Earth, over which we can spread lunar regolith to prevent radiation or micrometeorite damage.

louis · 2019-05-14 01:28:24

Interesting video from Curious Elephant on comparison of Falcon H9, SLS, and Starship (still referred to as BFR here) as rockets capable of delivering on NASA's lunar plans...

https://www.youtube.com/watch?v=gb1zNCpaA5w

He appears to state that the Starship can get there and back in one go.

Last edited by louis (2019-05-14 01:29:01)

GW Johnson · 2019-05-14 07:50:30

Louis:

It's not hard to verify what Starship/BFR can do, given some numbers appropriate to its weight statement and its engine performance. I already did this for myself and posted the results over at "exrocketman". I did it 3 times, starting with the larger 2017 version, then two iterations of the 2018 version, which is pretty close to what is being built now in 2019.

What you have to remember is that Starship and its big booster are first and foremost a transport from Earth's surface to low Earth orbit. It can take 100-ton class payloads to LEO without refueling. It requires an 1100-ton propellant load in the upper stage spacecraft to do that.

It can go other places if (and only if) you completely refill that spacecraft propellant load (1100 tons, mind you!!!) on orbit. Opinions differ, but all agree it takes 4 to 6 tanker flights to completely refill it on orbit. My own opinion is closer to 6 tanker flights. What has been published about that by Spacex is unclear at best. In particular, Musk has released nothing about the tanker design and its payload.

If refilled on-orbit fully, it can go to the surface of the moon with 100 ton-class payloads, and return with 50 ton class payloads without refilling. It cannot stop in lunar orbit on the way up, nor can it stop in Earth orbit on the way back. The mass ratio to support either of those is just not there. That's just rocket equation evaluations, with due regard to drag and gravity losses. The Earth landing is 99% aerobrake dissipation of kinetic energy, with a powered tail-first touchdown from a subsonic belly-flopping fall after entry.

If refilled fully on-orbit, it can go to the surface of Mars with 100-ton class payloads. Unless there is propellant manufacture sufficient for an 1100-ton refill on Mars, that trip is one-way. Period. It cannot stop in low Mars orbit on the trip there, there is way-far-insufficient mass ratio to support that.

Mars runs into it from behind, which is the least-expensive mission from a mass ratio requirement standpoint, forcing direct entry to the surface. Aerobraking dissipates about 99% of the kinetic energy, leading to a supersonic-to-subsonic tailslide maneuver in that super-thin "air" for the tail-first powered landing. Speed at entry interface is about 7.5 km/s, very comparable to entry interface from low Earth orbit (just under 8 km/s).

If refilled fully (1100-tons !!!), it can return to Earth with 50-ton class payloads. It cannot stop in low Earth orbit, there is not the mass ratio for that. It runs into the Earth from behind, and dissipates about 99% of the kinetic energy in aerobraking. The rest of the descent trajectory is exactly like the LEO return, just the speed at entry interface is higher. A lot higher. Something like 12-15 km/s.

If you look at the latest of my 3 postings about the capabilities of this design, you will find that they (Spacex) have yet to think their way through the rough-field landing problems (plural !!!). The landing legs are suitable for landing on a hard-paved pad, not rough dirt.

Spacex may initially fly it in this form, but it will not successfully go to the moon or Mars without some landing leg redesign. Even with that corrected, it is tall enough relative to its leg span to seriously risk tipover. Which would invariably be catastrophic and fatal. The same risks apply to emergency off-site rough-field landings on Earth, too.

GW

Last edited by GW Johnson (2019-05-14 08:10:27)

elderflower · 2019-05-14 08:54:16

That summary from GW makes me think of a mission to Titan. Methane is available in lakes and water takes the place of rocks. It would be much easier to refuel there provided you have the power sources.

Void · 2019-05-14 09:27:06

Elderflower, I think Elon Musk at least, and perhaps SpaceX as well is thinking that big for eventual use of the new launch hardware. Re-topping propellants at locations such as Mars, and just Oxygen at the Moon, or thereabouts.

GW Johnson, thanks for the sanity checks. You help us color inside the lines at least some of the time.

Due to the conversation of the members here, I have a trial balloon proposal. By no means to I think it is the absolute end of new ideas for the Moon, and the new hardware proposed, and actually being built towards.

But my proposal might have useful aspects to modify into even better plans.

OK, things I have discovered from talk for Elon Musk. Starship and Super Heavy, may be cheaper than Falcon 9 to build. This new launch device should according to Elon Musk, lift loads to LEO, at a cost >=1/10th that of Falcon 9. These things will prove true or not true. I guess it does matter, I presume that Elon Musk and SpaceX have grey matter much better than mine, so for now I will trust their projections.

So, some questions: "What will be the service life of a "Starship". How many flights? If you reach the maximum expected life of a "Starship" what do you do with it?

I am going to suggest that you retire them to orbit for space stations, or to the Moon, for the tankers, and perhaps Mars for the crew vehicles, and maybe at first at least for the cargo only Starships.

I don't see any reason to retire them to Earth for Scrap metal, that seems silly.

So on the last launch to LEO, maybe you cut down on the number of engines in the Starship, since it is never going to land in a 1 g field again. You only give it enough propellants to reach LEO, it can be refueled in orbit, to be able to reach other orbital locations/Luna/Mars/Ect. And on that last launch, probably you use old engines, and launch no significant cargo. I think that the days where engines can be removed in orbit or on the Moon and returned to Earth are far off or never, economically and practically.

So, now I am presuming that GW is correct about the landing gear for Starship. The only reason I might not, is that Elon Musk has said that it can land safely on the Moon. However, he may not be showing all of his cards, for good reason. For instance New Glen is going to have landing legs more like those of Falcon 9. I really think that we have to celebrate the use and congregation of "Best Practices". But of course SpaceX will want their pay days, and not be eager to tip their hand.

So, I will try coloring within the lines of defined proposals.

Blue Moon, other than eventually a potential crew carry, device, will start as a highly automated cargo delivery device. So to solve the problem or where a elderly Starship lands on the Moon as it's retirement objective, perhaps Blue Moon can deploy some useful devices such as a landing surface. And then I presume perhaps a tanker Starship lands on that pad. While I remember that GW thinks the Starship will need to be able to do emergency landings on any part of the Moon, I don't think that holds true for propellant Starships being delivered to their final resting place. The only concern there is you don't want a crash/collision/explosion event, that would damage much of the rest of the base.

Part of the game would be to calculate the safe amount of propellants to best assure a safe landing, and yet in the event of a crash, minimize the explosive forces. You would probably also include an abort to crash site (Scrap metals), some safe distance off, where the ship landing (without crew), would try to push itself to, in the event it was malfunctioning and placing the base at risk.

So, then a base. Perhaps to a large extent made of retired Starships, probably of the propellant lift type. You set up infrastructure for them to land on. They land, with robots, you heap regolith around the lower portions of them, as some type of radiation protection. This would by itself be pretty good near the south pole as the suns output would be somewhat perpendicular to the standing retired Starship.

But for splash radiation, and GCR, I suppose you then put some kind of "Roof Regolith" inside the retired ship at certain points. I don't think raw regolith would be good for that. Either sintered bricks, or glued bricks. You convert the purged propellant tanks to be additional pressurized space. Perhaps if there is unpressurized cargo holds, you might make them also capable of holding pressure.

So, now you have more lower portions of the retired Starship that are protected from radiation. You also have some part of the starship projecting above the regolith berm. Of course you are going to have an airlock, and perhaps a regolith ramp to access the lunar surface.
The portion of the ship projecting above the regolith can have a rotating solar collectors assembly with bearings attached to the nose of the ship, and drooping downwards towards the Lunar surface. The sun while available most of the time will always be on the horizon. This type of solar collector should be ideal, if it can be kept in working condition.

So, over time, there should be lots of Starships to retire either due to extended wear, or due to becoming obsolete per presumed new and better revisions. So, how big should the Lunar base be?

Well, at first, just a handful of science and experimentally oriented crew. Perhaps later even tourists, other scientists.

That would be for the case where you are trying to determine human health in the Lunar environment, experimenting on adaptations to the Moon, and discovering more basic science about the Moon.

But later, are you shipping water to LEO? Maybe more robots and humans on the Moon to support that.

The amount of water is unknown. If there is any, then if it is a smaller than expected amount, then use at the Moon base. If > than that, then propellants for things like the Blue Moon, and Oxygen for operative Starship vehicles, for a little while. If the water amount is limited and will not replenish at a sufficient rate from the solar wind striking the Moon, then at some point the water has to be conserved.

The latest theory of the formation of the Moon, however, suggests that the Moon was created in great part from liquid rock splashed off of the Earth by an impactor. My current understanding allows for volatile materials to be dissolved in such liquid rock. I am not convinced that it would necessarily all outgas immediately. It might have held into the lava sufficiently long to accrete into the subsurface of the Moon. Maybe. So, although the surface of the Moon is rather dry, where ancient volcanism exposed lava to the vacuum and solar wind, it may not be the same say a significant distance down. Perhaps there will be nice resources better than what the surface offers. We might find out eventually. Of course some rocks will have been ejected from that lower material. Not sure how much that possibility is supported by evidence. But not all of the Moon has been explored by a long shot.

The general trend since Apollo, has been to upgrade the Moon towards more Mars like, Earth like characteristics.

Elsewhere Taraformer, and Josh, have been investigating Lunar Nickle, and Iron. I myself am an advocate of Lunar Oxygen, not from water, but by leveraging Hydrogen, and other methods, to extract it from Lunar Rocks. I think that this could greatly support Starshp on trips to Mars, maybe Titan, and other objectives.

At some point a Lunar base may support trips by something like starship and other devices to NEO's.

That's about it for now. Hope I have not messed up your thought streams too much.

Done.

Last edited by Void (2019-05-14 10:14:40)

GW Johnson · 2019-05-14 13:44:03

There’s been some discussions on these forums about the new reveal of stainless steel construction for BFS/Starship. I think I know why they went to stainless steel, in spite of its weight.

Falcon-9 first stage cores are thin-wall uninsulated cryogen tanks of the lithium-aluminum alloy. This material is limited in the temperatures it can be exposed to, which are tolerable if a first stage entry at Earth is carefully done under precise control. It is just not survivable for full orbital entry speeds or higher.

Aluminum (lithium or other alloy) can be used to contain a mild cryogen (liquid oxygen, the bulk of the total propellant volume), and it is lightweight. But it has notoriously poor fatigue resistance. Going all the way from liquid oxygen temperatures to near-max exposure ~ 350 F) during first stage entry is a very stressful cycle. That means the fatigue life will be very, very short.

The latest incarnation of the Falcon cores will exceed 2 reflights, earlier versions did not fly more than twice. I would be very surprised indeed, if Falcon cores ever fly more than roughly 10 times, because of the fatigue cycling effect. Sooner or later, it will simply crack open upon filling the liquid oxygen. And that would be catastrophic.

The best cryogen tanks in the chemical industry are made of 300-series stainless steel. Usually 304L, because welding is needed (you cannot successfully weld plain 304, it will soon crack at the weld). Fatigue due to cycling hot and cold is not so much of an issue, especially compared to aluminum. 304/304L has a max service temperature rating near 1200 F, limited by permanent surface corrosion effects (scaling, among other symptoms).

There are other stainless materials that will serve even hotter, such as 316L, 347, and 309/310. 1600 F is achievable repeatedly without permanent corrosion, and in the case of 309/310, 1900 F is achievable. None of these have much strength at material temperatures above 1000 F: something under 2-3 ksi, weaker the hotter you soak it. In comparison, aluminum has almost zero strength at 350 F, and is a white hot puddle at 900-1000 F. 304L, and 316L are cheaper and quite readily available. 347 is available but expensive. 309/310 is not easily available, and very expensive.

During an entry at LEO conditions, there will be parts of the leeward side of the BFS/Starship airframe that could be bare stainless re-radiating heat to the environment. This might work at low emissivity (at or under 0.20), but would work a lot better if the surface had a high emissivity (at or above 0.80).

There would be no real possibility of reradiation cooling on the windward side, because quite a bit of strength is required to resist the wind pressure loads (500-5000 lbf/sq.ft). That material must be kept a lot cooler: under ~ 800 F. It just inherently takes a lowered-conductivity heat shield to do that.

All that is why I think the windward side and leading edges/nosetip of BFS/Starship will be covered with PICA-X ablative, with a dark black surface for highest-feasible emissivity. The leeward lateral surfaces might be exposed stainless for entry from LEO, but will have to be “blackened” for high emissivity coming back from the moon or Mars.

Those Earth entries from the moon or Mars will exceed 10 km/s at entry interface, such that considerable plasma radiation heating adds to the convective heating. While convective heating varies roughly as velocity squared, plasma radiation heating varies as velocity ^ 6, and quickly dominates above ~ 10 km/s.

The published illustrations about the BFS/Starship design show a nested tankage arrangement. The big volume seems to be thin wall uninsulated tankage. The small volume nested within looks to be a more insulated design. I cannot really tell just how insulated, though.

My interpretation says the departure propellant is the bigger outer volume, and it is all empty after departure. The smaller volume nested within contains propellant for course corrections and propulsive landing. Thus the larger outer volume is a sort of heat protection shield for the inner tankage.

The empty outer tank is part of the total heat protection/insulation package for the inner small tankage. The inner tankage never sees the entry heating, the outer tank wall does instead. And it is essentially unpressurized when it does: thus only the externally-applied entry airloads need be resisted structurally by that outer tank shell.

Those airloads are trivial on the leeward side, and so that portion can be allowed to soak up to the service limits at little or no strength. The windward side needs significant strength to resist the airloads, and so must be maintained below 800 F, perhaps well below. Which is why the heat shield material is utterly required.

Entry is a brief transient. Fundamentally, you limit the heat flow into the interior, while maintaining survivable material temperatures. It’s a heat sink design approach: you soak it up, and dump it after landing. Steady supersonic/hypersonic flight is quite different. If you heat sink, flight times must be short (seconds to a few minutes).

If you fly for long times, it’s essentially the steady state problem: every heat energy unit you absorb must be dumped. The only place to do that is out the engine tailpipe. It is fundamentally a much harder problem, as at those speeds, there is simply no such thing as cooling air.

GW

elderflower · 2019-05-15 14:01:23

Does the small inner tank not contain helium for pressurisation of the outer, GW? This would explain the need for insulation against heat transfer from the LOX or Methane in the outer.

louis · 2019-05-15 16:32:26

Do you really need to deliver 100 tons to the lunar surface every time? Perhaps for the first few missions, to build your base.

But thereafter, if you were taking say 10 tourists a time to the surface would you need a full tank? So maybe 10 + 5 crew...It's a 3-4 day trip. So you don't necessarily need huge supplies on board. Your lunar hotel base can grow food on the lunar surface. With efficient on board recycling of water and breathing gases, maybe you need take no more than 10 tons or so as your payload.

So could you take 10 tons to and back from the lunar surface, with say a half full or quarter full tank?

GW Johnson wrote:

Louis:
It's not hard to verify what Starship/BFR can do, given some numbers appropriate to its weight statement and its engine performance. I already did this for myself and posted the results over at "exrocketman". I did it 3 times, starting with the larger 2017 version, then two iterations of the 2018 version, which is pretty close to what is being built now in 2019.
What you have to remember is that Starship and its big booster are first and foremost a transport from Earth's surface to low Earth orbit. It can take 100-ton class payloads to LEO without refueling. It requires an 1100-ton propellant load in the upper stage spacecraft to do that.
It can go other places if (and only if) you completely refill that spacecraft propellant load (1100 tons, mind you!!!) on orbit. Opinions differ, but all agree it takes 4 to 6 tanker flights to completely refill it on orbit. My own opinion is closer to 6 tanker flights. What has been published about that by Spacex is unclear at best. In particular, Musk has released nothing about the tanker design and its payload.
If refilled on-orbit fully, it can go to the surface of the moon with 100 ton-class payloads, and return with 50 ton class payloads without refilling. It cannot stop in lunar orbit on the way up, nor can it stop in Earth orbit on the way back. The mass ratio to support either of those is just not there. That's just rocket equation evaluations, with due regard to drag and gravity losses. The Earth landing is 99% aerobrake dissipation of kinetic energy, with a powered tail-first touchdown from a subsonic belly-flopping fall after entry.
If refilled fully on-orbit, it can go to the surface of Mars with 100-ton class payloads. Unless there is propellant manufacture sufficient for an 1100-ton refill on Mars, that trip is one-way. Period. It cannot stop in low Mars orbit on the trip there, there is way-far-insufficient mass ratio to support that.
Mars runs into it from behind, which is the least-expensive mission from a mass ratio requirement standpoint, forcing direct entry to the surface. Aerobraking dissipates about 99% of the kinetic energy, leading to a supersonic-to-subsonic tailslide maneuver in that super-thin "air" for the tail-first powered landing. Speed at entry interface is about 7.5 km/s, very comparable to entry interface from low Earth orbit (just under 8 km/s).
If refilled fully (1100-tons !!!), it can return to Earth with 50-ton class payloads. It cannot stop in low Earth orbit, there is not the mass ratio for that. It runs into the Earth from behind, and dissipates about 99% of the kinetic energy in aerobraking. The rest of the descent trajectory is exactly like the LEO return, just the speed at entry interface is higher. A lot higher. Something like 12-15 km/s.
If you look at the latest of my 3 postings about the capabilities of this design, you will find that they (Spacex) have yet to think their way through the rough-field landing problems (plural !!!). The landing legs are suitable for landing on a hard-paved pad, not rough dirt.
Spacex may initially fly it in this form, but it will not successfully go to the moon or Mars without some landing leg redesign. Even with that corrected, it is tall enough relative to its leg span to seriously risk tipover. Which would invariably be catastrophic and fatal. The same risks apply to emergency off-site rough-field landings on Earth, too.
GW

GW Johnson · 2019-05-15 16:34:02

Elderflower:

Musk's presentation on their website refers to these as "header tanks", and specifically states that they hold the landing propellant during transit, because the landing propellant sloshing around in the big volume tankage is unacceptable.

The configuration in that presentation is the same 9 m dia and 48 m long dimension, but with the earlier small fixed delta wing and 4 extendible landing legs.

I doubt the inboard profile has changed much, but the fins have (to 3 with landing pads at their tips), and he now shows 7 engines of the sea level Raptor design elsewhere in the site materials, along with 80-some cubic meters of storage volume around them.

The slightly earlier delta wing design had 2 sea level engines and 4 vacuum engines for a total of 6.

GW

Last edited by GW Johnson (2019-05-15 16:38:38)

SpaceNut · 2019-05-15 17:53:45

I think a good part of tha 100 ton is water shield for lunar use.
The changing design also indicates that with real life testing the numbers are not what they were expecting....
The small tank for landing as well is easier to maintain pressure as well with the small volume....

louis · 2019-05-17 10:19:26

Can't recall now if anyone has posted this. The Blue Moon lander:

https://www.youtube.com/watch?v=D9EyCVb4HAw

Looks good and makes a lot of sense - using hydrogen as a propellant, coolant and power source for hydrogen fuel cells. Will be able to send by Gigabyte amounts of data via laser.

Last edited by louis (2019-05-17 14:52:58)

Void · 2019-05-17 13:47:10

Very encouraging. I think that Blue Origin and SpaceX will be encouraged to continue by the powers that be. There may be more contenders to come as well.

Done.

kbd512 · 2019-05-17 19:12:05

GW,

SpaceX could forego Starship's header tanks, with its attendant structural support within the structure of the main tanks, and simultaneously reinforce the walls of the main tank, by switching to a double-wall balloon tank design.

Starship's propellant tanks could also be filled with this stuff to prevent propellant sloshing:

A new graphene foam stays squishy at the coldest temperatures

The side walls of the tank could be protected from MMOD impacts with woven / braided CNT tapes. The interior of the tank could receive another layer. The plane in which heat is conducted is also adjustable by changing the direction of the CNT fibers. The thermal conductivity can either be exceptionally poor or exceptionally good, dependent upon which property is desired for a specific application.

GW Johnson · 2019-05-18 08:55:47

Kbd512:

What you say is true. I hope Spacex looked at all those design options. The problem with vehicle design optimization is that it is almost entirely constraint-driven. Whatever you think your constraints are almost totally drives your answer. That process/thinking has not been made public, and I doubt it ever will be.

My own opinion of the constraints applied here makes entry survival for the free Earth return the driving constraint. That will occur at 12-15 km/s speeds at entry, depending upon the orbital details of the two planets, and just how "fast" a transfer orbit can be flown.

This is way above the nominal 10 km/s speed at which plasma radiation dominates the picture. Peak heating rates will be far beyond even those for Apollo returning from the moon.

In order to use temperature-susceptible things like aluminum or organic composites for your structure, you absolutely must keep the material soakout temperatures under 350-200 F respectively. That absolutely requires a leeward-side refractive/ablative heat shield of lowered thermal conductivity.

The lowered thermal conductivity is achieved by lowered density of the heat shield material, at the cost of structural fragility. Yet it is still a large weight addition over just a bare aluminum or composite shell. Further, in the case of the composite material, there must be an impermeable liner, since the composite material is inherently porous. That's another weight gain, offsetting its advantage over aluminum to one extent or another.

For less temperature-susceptible metals like high-alloy or stainless steels, you can allow them to "float" into the effective re-radiation range of 1200 F+. That makes an exposed metal surface cooled during the peak heating transient largely by re-radiation, plus some conduction into the interior heat sink, whatever that is.

The downside with this design is hot structural weakness (you will never do this successfully on the windward side where heating and wind pressures are far higher, even just from LEO). The advantage is that the heavier shell weight disadvantage is offset by the weight advantage of not needing leeward-side heatshield, or impermeable liner materials.

This last (exposed leeward metal shell) works better at entry speeds well under 10 km/s where the plasma radiative heating is negligible. Such is the case for LEO entry (8 km/s) and direct-trajectory Mars entry (7.5 km/s per Spacex). Such is not the case for Earth free return from Mars, so my personal opinion is that this heat protection design of Spacex's has more evolving to do (just like their rough-field landing capability has a lot more evolving to do).

The other design constraint that Musk indicated in his 2018-vintage slide presentation is no propellant slosh inside the tanks during transit. He said so very explicitly in that presentation. The propellant needed for landing is far smaller in volume than the main tankage volume.

Containing it within smaller "header tanks" nested inside the main tanks achieves satisfying that constraint without the weight gain and volume losses of baffles and foam, plus it allows these smaller header tanks to feature some insulation to reduce evaporation losses during months of transit. This allows you to skip any evaporation control gear or insulation on the main outer tanks with their far larger surface areas.

All in all, I think their designs are fairly-well thought-out, as far as they go. As I have said before, that thinking process is not yet over (it doesn't have to be for the initial prototypes). Bear in mind that first-and-foremost, this thing is a transport to and from LEO, not a deep space vehicle design. With refilling on orbit, it can be used as a deep space vehicle.

GW

Last edited by GW Johnson (2019-05-18 09:08:53)

kbd512 · 2019-05-18 12:16:54

GW,

I'm not advocating for using composites here. I'm saying that the various CNT ribbons, tapes, and foams can be used to insulate the tank, provide MMOD protection, and provide propellant slosh baffling. Aerographene foam would weigh 600kg for a quantity of foam that would fill the entire SLS LH2 tank. Would stainless steel header tanks and their support structure in Starship weigh less than 600kg? Given their size, I kinda doubt it, but I'm curious to know.

GW Johnson · 2019-05-21 08:27:55

Kbd512:

Sorry I didn't reply sooner, I was in the hospital the last 3 days.

You probably know more than me about the newer foam materials. I haven't kept up with it. 50-60 years ago, there was a urethane foam developed to prevent slosh and prevent fire-spread in aircraft drop tanks. It worked great, and cost you about 3% of internal volume. No explosion in a gasoline drop tank when hit by 50 caliber tracer fire, just a quiet burn.

There are two problems to solve here: one is slosh, the other is keeping entry heat away from the on-board propellant. The header tank solves both. If the outer tank is empty of liquids, that void space is excellent insulation to keep the heat at the hot outer wall away from the inner header tank. The very "best" insulation would be vacuum in that outer tank space, but vapors have very low thermal conductivity, and free convection is the weakest of all the convective heat transfer processes.

The only problem with the outer tank / inner header tank approach is survival of that outer tank during an entry event. If a thin wall metal shell, there is limited heat capacity to heat sink your way through the fundamentally-transient heating spike of entry. That makes high emissivity for efficient re-radiation utterly crucial for survival. And THAT means a spectrally-black surface at the thermal emission wavelengths. Not the same as visible wavelengths, but it does overlap some.

In one of the other threads, Louis was talking about 6 cm resolution capability, which I presume is photography from one of the orbiting probes at Mars. He was trying to make a case that this "proves" there are safe places to land a Spacex Starship, with its current landing pad design sizing.

That's a very impressive photographic resolution, but photography is of the surface, and says absolutely nothing about what's beneath. Given the current state of humanity's technologies, the ONLY way to find out what's beneath the Martian surface is real ground truth. You have to go there and dig or drill.

His other point was based on some kind of definitional argument. Something about landforms being classified as "exposed rock platforms". Trouble with that is, anything definitional is just a best guess based on surface appearances. Ground truth trumps all. Best to be prepared to land in soft sand, which a lot of Mars seems to be. Then you can land anywhere, and not be restricted by local soil types.

For the Spacex Starship design, touchdown is less of a problem than re-launch after refilling. The refilled weight, even on Mars, is about 6 times the touchdown weight. You don't want it sinking into the dirt (and tipping over), while you refill it with propellants.

I've got those numbers posted over at "exrocketman" some time ago, along with a listing of various Earthly soil strengths I got out of an older edition Marks' Mechanical Engineer's Handbook. Doesn't matter that it's an old reference. Those numbers haven't changed in over a century.

GW

Last edited by GW Johnson (2019-05-21 08:41:12)

SpaceNut · 2019-05-21 16:59:09

Sorry to hear that GW, I hope all will be well..

I tried as well to point the image issue but had not covered anything for radar as thats not going to be able to tell much as I do not believe we can get that sort of depth measurements and then look even deeper with each pass of the probe over head so as to create a void detection let alone tell if its solid or soft....

kbd512 · 2019-05-21 21:17:01

GW,

I hope you're feeling better now.

New Mars Forums

Announcement

#151 2019-03-19 17:00:37

Re: Lunar economics etc

#152 2019-03-20 03:38:01

Re: Lunar economics etc

#153 2019-03-20 09:42:17

Re: Lunar economics etc

#154 2019-03-20 19:14:49

Re: Lunar economics etc

#155 2019-05-10 20:10:06

Re: Lunar economics etc

#156 2019-05-10 21:43:34

Re: Lunar economics etc

#157 2019-05-11 08:22:13

Re: Lunar economics etc

#158 2019-05-11 08:38:36

Re: Lunar economics etc

#159 2019-05-14 01:28:24

Re: Lunar economics etc

#160 2019-05-14 07:50:30

Re: Lunar economics etc

#161 2019-05-14 08:54:16

Re: Lunar economics etc

#162 2019-05-14 09:27:06

Re: Lunar economics etc

#163 2019-05-14 13:44:03

Re: Lunar economics etc

#164 2019-05-15 14:01:23

Re: Lunar economics etc

#165 2019-05-15 16:32:26

Re: Lunar economics etc

#166 2019-05-15 16:34:02

Re: Lunar economics etc

#167 2019-05-15 17:53:45

Re: Lunar economics etc

#168 2019-05-17 10:19:26

Re: Lunar economics etc

#169 2019-05-17 13:47:10

Re: Lunar economics etc

#170 2019-05-17 19:12:05

Re: Lunar economics etc

#171 2019-05-18 08:55:47

Re: Lunar economics etc

#172 2019-05-18 12:16:54

Re: Lunar economics etc

#173 2019-05-21 08:27:55

Re: Lunar economics etc

#174 2019-05-21 16:59:09

Re: Lunar economics etc

#175 2019-05-21 21:17:01

Re: Lunar economics etc

Board footer